The Mo of the MoFe protein of nitrogenase is being examined by XAS (X-ray absorption) and EXAFS (extended X-ray absorption fine structure). Based on background information from model synthetic molybdenum compounds, it is possible to conclude that the Mo of the MoFe protein has a charge of plus 3 or plus 4, that it has sulfur ligation, and that it has Fe closely associated with it. In addition it is possible to rule out the presence of Mo equals 0 bonding. Preliminary results with both the dye oxidized and fully reduced MoFe proteins of nitrogenase suggest that the Mo does not undergo changes in its oxidation state during these reactions. However, the Mo of O2-oxidized MoFe protein (inactive protein) does show changes in oxidation state as well as the presence of oxygen ligands. It is proposed based on these results that a model for the Mo center of nitrogenase could be the replacement of one of the Fe atoms of a Fe4S4 cluster by Mo. Research is in progress to determine the effect of nitrogenase substrate on the Mo XAS and EXAFS. A second Mo-containing protein of Clostridium pasteurianum, synthesized only when this organism is fixing dinitrogen, presumably functions in the synthesis of the Mo center of the MoFe protein. Preliminary EXAFS data suggests that its Mo does have oxygen ligands. Its role in nitrogenase synthesis is being sought.